DESCRIPTION

EVP_DigestInit_ex() sets up digest context ctx to use a digest
type from ENGINEimpl. ctx must be initialized before calling this
function. type will typically be supplied by a functionsuch as EVP_sha1().
If impl is NULL then the default implementation of digest type is used.

EVP_DigestUpdate() hashes cnt bytes of data at d into the
digest context ctx. This function can be called several times on the
same ctx to hash additional data.

EVP_DigestFinal_ex() retrieves the digest value from ctx and places
it in md. If the s parameter is not NULL then the number of
bytes of data written (i.e. the length of the digest) will be written
to the integer at s, at most EVP_MAX_MD_SIZE bytes will be written.
After calling EVP_DigestFinal_ex() no additional calls to EVP_DigestUpdate()
can be made, but EVP_DigestInit_ex() can be called to initialize a new
digest operation.

EVP_MD_CTX_cleanup() cleans up digest context ctx, it should be called
after a digest context is no longer needed.

EVP_MD_CTX_destroy() cleans up digest context ctx and frees up the
space allocated to it, it should be called only on a context created
using EVP_MD_CTX_create().

EVP_MD_CTX_copy_ex() can be used to copy the message digest state from
in to out. This is useful if large amounts of data are to be
hashed which only differ in the last few bytes. out must be initialized
before calling this function.

EVP_DigestInit() behaves in the same way as EVP_DigestInit_ex() except
the passed context ctx does not have to be initialized, and it always
uses the default digest implementation.

EVP_DigestFinal() is similar to EVP_DigestFinal_ex() except the digest
contet ctx is automatically cleaned up.

EVP_MD_CTX_copy() is similar to EVP_MD_CTX_copy_ex() except the destination
out does not have to be initialized.

EVP_MD_size() and EVP_MD_CTX_size() return the size of the message digest
when passed an EVP_MD or an EVP_MD_CTX structure, i.e. the size of the
hash.

EVP_MD_block_size() and EVP_MD_CTX_block_size() return the block size of the
message digest when passed an EVP_MD or an EVP_MD_CTX structure.

EVP_MD_type() and EVP_MD_CTX_type() return the NID of the OBJECTIDENTIFIER
representing the given message digest when passed an EVP_MD structure.
For example EVP_MD_type(EVP_sha1()) returns NID_sha1. This function is
normally used when setting ASN1 OIDs.

EVP_MD_CTX_md() returns the EVP_MD structure corresponding to the passed
EVP_MD_CTX.

EVP_MD_pkey_type() returns the NID of the public key signing algorithm associated
with this digest. For example EVP_sha1() is associated with RSA so this will
return NID_sha1WithRSAEncryption. This ``link'' between digests and signature
algorithms may not be retained in future versions of OpenSSL.

EVP_dss() and EVP_dss1() return EVP_MD structures for SHA and SHA1 digest
algorithms but using DSS (DSA) for the signature algorithm.

EVP_md_null() is a ``null'' message digest that does nothing: i.e. the hash it
returns is of zero length.

EVP_get_digestbyname(), EVP_get_digestbynid() and EVP_get_digestbyobj()
return an EVP_MD structure when passed a digest name, a digest NID or
an ASN1_OBJECT structure respectively. The digest table must be initialized
using, for example, OpenSSL_add_all_digests() for these functions to work.

RETURN VALUES

EVP_DigestInit_ex(), EVP_DigestUpdate() and EVP_DigestFinal_ex() return 1 for
success and 0 for failure.

EVP_MD_CTX_copy_ex() returns 1 if successful or 0 for failure.

EVP_MD_type(), EVP_MD_pkey_type() and EVP_MD_type() return the NID of the
corresponding OBJECTIDENTIFIER or NID_undef if none exists.

EVP_get_digestbyname(), EVP_get_digestbynid() and EVP_get_digestbyobj()
return either an EVP_MD structure or NULL if an error occurs.

NOTES

The EVP interface to message digests should almost always be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the digest used and much more flexible.

SHA1 is the digest of choice for new applications. The other digest algorithms
are still in common use.

For most applications the impl parameter to EVP_DigestInit_ex() will be
set to NULL to use the default digest implementation.

The functions EVP_DigestInit(), EVP_DigestFinal() and EVP_MD_CTX_copy() are
obsolete but are retained to maintain compatibility with existing code. New
applications should use EVP_DigestInit_ex(), EVP_DigestFinal_ex() and
EVP_MD_CTX_copy_ex() because they can efficiently reuse a digest context
instead of initializing and cleaning it up on each call and allow non default
implementations of digests to be specified.

In OpenSSL 0.9.7 and later if digest contexts are not cleaned up after use
memory leaks will occur.

EXAMPLE

This example digests the data ``Test Message\n'' and ``Hello World\n'', using the
digest name passed on the command line.